Help Me Diagnose Please

I can't see any of the white stringy stuff typical of fungus. Most of what is called fungus is actually caused by bacteria and the treatment is with an antibacterial rather than an antifungal.

This is the fungal disease. Notice the article mentions strands growing from a body wound.
But this is what most people mean by fungus, caused by bacteria.
Have a read of the two articles, especially the sections on symptoms. If you think it looks more like columnaris that saprolegnia, then you need an anti bacterial. The best two available are Myxazin by Waterlife and eSHa 2000. But you need to remove the anti fungal first and the easiest way to do this is a large water change to remove most of it then run carbon in the filter for at least 24 hours to remove the rest.
I keep a small, cheap internal filter in the cupboard for when I need to use carbon. I wrap the carbon granules in a piece of net fabric and put that inside the filter, being careful that the loose ends don't get into the impeller well.

I would switch over straight away, and use the carbon pads. Throw them away when their job is done.